PCR analysis has characterized tissue-specific expression of CD44 isoforms resulting from alternative splicing of up to 10 exons. To examine isoform expression at the level of cell surface protein and to probe the possible biological functions of CD44 isoform products in vitro or in vivo, we have generated monoclonal antibodies specific for variable exon products. Immunization with fusion proteins containing variable CD44 exon products produced hybridomas with multiple CD44 variable exon specificities. Flow and immunohistochemical techniques detected variable exon determinants on epithelial cells and on activated lymphocytes. mAbs and fusion proteins are being used to assess the function of CD44 variable isoforms. To identify functional cell surface molecules expressed on activated lymphocytes, mAb were generated by immunizing rats with activated mouse B cells. One of these mAb (GL7) reacts with subpopulations of activated B and T cells, as well as on a functionally distinct subpopulation of CD4+8- thymocytes with unique cytokine secretory capacity and on bone marrow pre-B cells. GL7 administered in vivo influences early B cell development. Expression cloning identified the determinant recognized by GL7 as an alpha 2,6-sialyl transferase-dependent epitope(s) present on human as well as mouse lymphoid cells. Characterization of the activation antigen-specific mAb GL1 led to identification of the mouse B7-2 costimulatory molecule, which has a predominant functional costimulatory role both in vivo and in vitro. Anti-B7-2 mAb inhibited accessory cell-dependent responses of T cells in vitro and in vivo, indicating that B7-2 is a functional costimulatory molecule for T cell-dependent (TD) responses. Expression of B7-2 costimulatory molecules during in vivo TD antibody responses correlated with somatic hypermutation and affinity maturation, and in vivo treatment with anti-B7-2 inhibited TD responses as well as memory formation and somatic mutation. The role of B7-1 and B7-2 in tumor rejection was analyzed using tumor cells transfected with B7-1 or B7-2. Both B7-1 and B7-2 induced tumor injection, and the cytoplasmic domains of these B7 molecules were not required. An essential role of host T cells and of host CD28 was shown by the failure of athymic or CD28- deficient mice to reject B7-transfected tumors.